Microwave oven cavities or cabinets are presently constructed of steel. In order to reduce the cost of the overall microwave oven, it is clearly desirable to construct the cavity using a suitable plastic material such as an injected molded plastic or polymeric material as the material of construction. Such plastics are, however, non-conducting and permit the escape of microwave energy by transmission outside of the cavity. For safety to the user and for proper adjustments of the microwave oven, this is highly undesirable. It is, therefore, clearly necessary to provide some form of conductor to provide a shielding and cavity "tuning" function.
Many different methods designed to realize a conductor on the outside of a plastic microwave cavity have been attempted. For example, a fine mesh metal screen has been incorporated in the plastic layer and does provide the desired shielding function, but this procedure is not amenable to plastic molding processes. Loading, i.e., filling, the plastic with a conductive material has also been attempted but in such constructions, each metallic particle is insulated and isolated in the plastic matrix resulting in significant heating losses. Covering the outside of the microwave cavity with a conductive paint does provide the required shielding function; unfortunately, such conductive paints are based on a polymer which is loaded with a silver powder which means that the costs fluctuate greatly depending on the price of silver available on the world market and at present prices, the silver conductive coatings eliminate any cost advantages realized by using plastic cavities instead of steel. Further, while it is feasible to load such polymers with carbon in order to provide the conductive function at a lower cost, the resistivity is much too high to provide adequate shielding and tuning.
Accordingly, it is the object of this invention to provide a shield coating process for a plastic microwave oven cavity to provide a coating of sufficiently low resistivity to be effective for microwave shielding, which process is facile, low in cost and provides a shield coating which is contiguous without breaks at the edges of the cavity or holes in the conductive coating. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.